jonik
Scoot Member
Posts: 58
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Post by jonik on Sept 6, 2010 5:41:18 GMT -5
Was normally driving yesterday and all of sudden, there was half of the compression gone. The engine never overheated or such. Just checked what happened. Piston ring was in bad shape and there were seize marks due to overheating on the other side of the piston. It is kinda interesting that all the marks are there, from were the hot air comes out from the shroud. So now I need to create something to prevent that from happening again. I drived 750km with that piston, it looks almost new from elsewhere. I hone the cylinder before new piston arrives. I use 3% mixture with full synthetic oil. There was also another thing which I noticed. Piston head has 0,5mm hollow on the middle. I know that when you have too hot plug it will make a hole into your piston head. I use NGK BR9HS, that shouldn't be too hot. Maybe I try BR10HS next time.  jonik |
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Post by 190mech on Sept 6, 2010 6:37:34 GMT -5
OUCH!It looks like you have a lean condition there,might be an air leak from a crank seal or reed or base gasket leak.Please pressure test it when you reassemble it..Part throttle leaning is a problem with a tuned/piped scoot.The needle may need to be set richer or perhaps a thinner needle for part throttle..
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jonik
Scoot Member
Posts: 58
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Post by jonik on Sept 6, 2010 7:53:09 GMT -5
 Here's the plug. I don't have tools for pressure test (only for cylinder), but everything should be ok. The carb is adjusted properly, needle etc. Air leak may be the reason for this. I'll check the reeds next. jonik |
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Post by 90GTVert on Sept 6, 2010 10:58:58 GMT -5
I don't think I'd worry about the cooler plug. If the plug is too hot, I would think it should be burning cleaner. It looks like the plug is showing that you have plenty of oil, but from the piston I would say not necessarily enough fuel.
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jonik
Scoot Member
Posts: 58
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Post by jonik on Sept 6, 2010 14:29:39 GMT -5
Thanks for help.
My block is now apart, switching crankshaft. The new crank is "wrong one" sent from scootparts.nl. It has 10mm piston pin. Now I just order piston with 10mm pin so I can use that. I have also stroker crank waiting for next build (42mm stroke). Problem is to find shop which will machine carters.
jonik
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Post by 90GTVert on Sept 6, 2010 15:20:42 GMT -5
Wow. You don't mess around. Burn a piston, replace te crank! ;D Good luck on the stroker build, keep us up to date on it. Machining the cases is pricey, but the bigger cc engines sure do move.
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jonik
Scoot Member
Posts: 58
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Post by jonik on Sept 7, 2010 9:23:14 GMT -5
The block is getting new paint on weekend, shiny red. I fiberglassed the shrouds, only the hot air output is still in progress. Hard to find correct shape and size. I'm still worried about that dent on the piston head. It's only happened with my AC engines. Pics from while back   I'll tell you a story about that right one. I had AC on my racing machine. I was testing top speed and suddenly all compression was gone. The reason was cooling fan or atleast what was left from it. Scooter should be up and running within two weeks. jonik |
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Post by 90GTVert on Sept 7, 2010 11:24:10 GMT -5
I've never seen the needle bearing go like the one in your pic.  I really think it's a lean issue. All of those pistons show signs of overheating other than the extremely clean one. The pinhole in the piston that looks new is pretty peculiar though. I dunno, I quit messing with aluminum cylinders just because they have such a tendency to burn pistons compared to cast iron setups. |
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jonik
Scoot Member
Posts: 58
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Post by jonik on Sept 7, 2010 13:26:37 GMT -5
Those pistons except new looking one are from my racing engine. Every time the carb settings were perfect or little too rich. After switching to LC there are no issues occured. I can drive 10km easily with full throttle and theres no problems. With AC even two kilometers on a hot day was a miracle. AC isn't just enough for 13500rpm vehicle.
jonik
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Post by 190mech on Sept 7, 2010 19:50:41 GMT -5
After looking at your photos carefully it appears the hole in the piston is from a loose piston pin needle bearing!Ive heard of the race guys going to 12mm pins to cure this problem..I guess thats the plan here too?
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jonik
Scoot Member
Posts: 58
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Post by jonik on Sept 8, 2010 0:03:31 GMT -5
Good theory but that can't be possible. That piston is from my daily drive scoot, it doesn't have broken down any bearings. That was my first though too.
On Racing engine, I have Motoforce Racing Evolution crank with 12mm piston pin. On my daily, I had original crank with 12mm piston pin and I'm now switching full circle crack with 10mm piston pin (I ordered 12mm but got 10mm one...). Tried to sell it months but no success. The new crank has Stage6 reinforced extra wide needle bearing, so it should work fine.
jonik
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Post by 190mech on Sept 8, 2010 4:34:33 GMT -5
That looks clearly like a pierced hole,does the cylinder head show any damage?
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jonik
Scoot Member
Posts: 58
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Post by jonik on Sept 8, 2010 5:05:42 GMT -5
There is no damage on the cylinder head. That piston has dent on the piston head also. Would be nice to know where that comes from. Lowering the compression didn't do anything.
All the time when I open the cylinder, I have to change the heads silicon O-ring. It's allways at bad shape. One time there was't anything left from it.
Can wrong ignition timing do this? I have MVT Millenium ignition. It's not originally adjustable but I made it one. It's on original timing though. It can now be adjusted to +-10 degrees. I'll try 3 degree advance next.
jonik
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jonik
Scoot Member
Posts: 58
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Post by jonik on Sept 21, 2010 14:50:25 GMT -5
Hi!
Parts arrived today, BUT the belt was wrong. I have to stick with the old one.
I increased the direct flow a bit. What kind of material do you use to stuff the carters? Chemical metal isn't strong enough.
jonik
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I really think it's a lean issue. All of those pistons show signs of overheating other than the extremely clean one. The pinhole in the piston that looks new is pretty peculiar though. I dunno, I quit messing with aluminum cylinders just because they have such a tendency to burn pistons compared to cast iron setups.